Pressure at points on the same horizontal plane

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Discussion Overview

The discussion revolves around the concept of pressure in a fluid at two points, A and B, located on the same horizontal plane. Participants explore the implications of hydrostatic pressure, the effects of gravity, and the nature of pressure transmission in fluids.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that pressure at points A and B on the same horizontal plane should be equal, citing the principle of hydrostatic pressure.
  • Others argue that the pressure at A and B can be expressed as h2*ρg and h1*ρg respectively, leading to confusion about their equality when h1 does not equal h2.
  • One participant explains that pressure in a fluid is caused by gravity and does not exert a force sideways, suggesting that pressure does not spread laterally.
  • Another participant counters that fluids transmit pressure in all directions and that pressure at different points in a fluid should equalize.
  • A later reply emphasizes that pressure differences can exist due to varying depths and atmospheric pressure, but at the same depth, pressures should equalize.
  • Some participants discuss the implications of pressure in different environments, such as high altitudes, but note that this is not directly related to the core question of pressure at the same depth.
  • There is mention of how fluid dynamics would behave in a zero-gravity environment, suggesting that pressure would equalize throughout the fluid in such conditions.
  • One participant clarifies that the pressure at A includes contributions from the atmospheric pressure above it, which must be considered when comparing it to pressure at B.

Areas of Agreement / Disagreement

Participants express differing views on how pressure behaves in fluids, particularly regarding lateral pressure transmission and the conditions under which pressures at different points can be considered equal. There is no consensus on the implications of the differing heights h1 and h2 in relation to pressure equality.

Contextual Notes

Some participants highlight the importance of atmospheric pressure and the specific conditions of the fluid system being discussed, indicating that assumptions about depth and external pressures are critical to understanding the situation.

Misr
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As you know,pressure is the same at points on the same horizontal plane
Okay,suppose two points A,B in a fluid are on the same horizontal plane
[PLAIN]http://img197.imageshack.us/img197/1977/unledfhe.jpg
the pressure on both points is the same
but at A pressure=h2*ρg
pressure at B=h1ρg
and h1 is not equal to h2 as you see
could you explain this
 
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Pressure is rho g h right? So the water in the middle has pressure applied on it from atmosphere as well as the water above it, while the water to the left only has pressure applied to it by the water above it. Pressure does not spread laterally.

If you push on a piece of paper, the paper around the paper you pushed on feels some of the pressure exerted too. This is not the case with water. Each molecule is very small and has a lot of freedom to move. Imagine that each water molecule in the middle has some force exerted on it because of pressure. You think that this force might cause those molecules to push on molecules to the left and right, but this is not the case.

A simpler explanation is that the pressure in this case is caused by gravity, and gravity doesn't exert a force sideways.
 
Actually, fluids - liquids and gases - transfer whatever pressure applied to them in all directions...downwards, sideways, upwards...

The atmospheric pressure out on the yard is 1 atm...when you snick back in the porch with a ceiling only 10 feet high...is the pressure a lot less? No.

If the pressure in A was not the same as the pressure in B...you could dive to the bottom of the ocean, carve a hole on a wall and a bit of ceiling to snick under and all of a sudden have minimal pressure? I don't think so.
 
Pressure is rho g h right? So the water in the middle has pressure applied on it from atmosphere as well as the water above it, while the water to the left only has pressure applied to it by the water above it. Pressure does not spread laterally.

If you push on a piece of paper, the paper around the paper you pushed on feels some of the pressure exerted too. This is not the case with water. Each molecule is very small and has a lot of freedom to move. Imagine that each water molecule in the middle has some force exerted on it because of pressure. You think that this force might cause those molecules to push on molecules to the left and right, but this is not the case.

A simpler explanation is that the pressure in this case is caused by gravity, and gravity doesn't exert a force sideways.

Actually, fluids - liquids and gases - transfer whatever pressure applied to them in all directions...downwards, sideways, upwards...

The atmospheric pressure out on the yard is 1 atm...when you snick back in the porch with a ceiling only 10 feet high...is the pressure a lot less? No.

If the pressure in A was not the same as the pressure in B...you could dive to the bottom of the ocean, carve a hole on a wall and a bit of ceiling to snick under and all of a sudden have minimal pressure? I don't think so.
so what is the pressure in the case above?
 
Pressure is rho g h right? So the water in the middle has pressure applied on it from atmosphere as well as the water above it, while the water to the left only has pressure applied to it by the water above it. Pressure does not spread laterally.

If you push on a piece of paper, the paper around the paper you pushed on feels some of the pressure exerted too. This is not the case with water. Each molecule is very small and has a lot of freedom to move. Imagine that each water molecule in the middle has some force exerted on it because of pressure. You think that this force might cause those molecules to push on molecules to the left and right, but this is not the case.

A simpler explanation is that the pressure in this case is caused by gravity, and gravity doesn't exert a force sideways.

Actually, fluids - liquids and gases - transfer whatever pressure applied to them in all directions...downwards, sideways, upwards...

The atmospheric pressure out on the yard is 1 atm...when you snick back in the porch with a ceiling only 10 feet high...is the pressure a lot less? No.

If the pressure in A was not the same as the pressure in B...you could dive to the bottom of the ocean, carve a hole on a wall and a bit of ceiling to snick under and all of a sudden have minimal pressure? I don't think so.
so what is the pressure in the case above?
 
Actually, fluids - liquids and gases - transfer whatever pressure applied to them in all directions...downwards, sideways, upwards...
so the extra pressure at point B is transferred to point A until they are equal in pressure??

The atmospheric pressure out on the yard is 1 atm...when you snick back in the porch with a ceiling only 10 feet high...is the pressure a lot less? No.
but the pressure is much less at high mountains
 
The pressures at A and B are the same.

Whether pressure is less at high mountains has not much to do with the problem at hand...you are talking about pressure between two points at the same depth inside a fluid.

But, yes, pressure at high mountains is less because such point is less deep measuring from the top of the atmosphere, down.
 
Ok what makes pressure the same at A and B?
 
Gravity doesn't need to "act sideways" to produce a pressure to the side any more than a crank lever needs to be pushed in the same direction as the direction you want a force to be applied. Fluid will flow from a high pressure region to a low pressure region - in any direction. If you did the experiment out in zero g, the pressures would all be equal everywhere in the container (cabin pressure or more if you squeeze on a sealed container). Back on Earth, there is a hydrostatic effect that causes the pressure at any level to increase with depth. This pressure will be acting in all directions so fluid will move (microscopically) from B towards A until equilibrium is reached. Then the pressure will be the same over all that particular level. If you were, suddenly, to add another metre of liquid to the vertical tube, there would be a slight delay before the wave of increased pressure traveled from B to A and equilibrium was again reached. (Speed of sound in the liquid)
 
  • #10
Misr said:
As you know,pressure is the same at points on the same horizontal plane
Okay,suppose two points A,B in a fluid are on the same horizontal plane

the pressure on both points is the same
True.
but at A pressure=h2*ρg
Not true.
pressure at B=h1ρg
True (neglecting atmospheric pressure).
and h1 is not equal to h2 as you see
could you explain this
h1 is the depth below an open surface, but h2 is not. The upper surface of the container above A exerts a downward pressure that must be added to h2*ρg to get the actual pressure at A. The net effect is that the pressure is equal at A and B.
 
  • #11
Yeah,that's great and easy to imagine
thanks very much
 

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